Disagree.
The behaviour of TimeConstraint.extract is distinct : We have effectively changed it specifically for this class, so it merits a test here.

I think the real problem is that \_CIM_extract is not a "properly" private method.
For example, TimeConstraint re-implements it.  If you wanted to create a Constraint subclass *outside* of this source module, that would not be allowed.  But, as we see here, that **is** the obvious means to implement a new Constraint type.
So possibly, \_CIM_extract "ought not" to be private ?  FWIW, I think in Java this would be a "protected" method, meaning it is not in the external api, but is available to *inheritors* - http://www.tutorialspoint.com/java/java_access_modifiers.htm

I discussed this with @bblay, and he suggested that making \_CIM_extract public (or some other refactoring) might be appropriate, but is certainly not in this scope.